1. Field of the Invention
The present invention relates generally to an method for dispersing sediment, such as hydrocarbon sludge, in a storage tank, and more particularly to such a method that provides repair of the dispersion apparatus without emptying the storage tank.
2. Description of the Related Art
It is a common commercial practice to store liquid materials in storage tanks. Typically, for many industrial applications, storage tanks will have a diameter from 100 to 300 feet and heights of 20 to 50 feet or more. The liquids stored in such storage tanks are diverse. For example, water or aqueous solutions of organic or inorganic chemicals may be stored in this manner, derivatives of agricultural products such as vegetable oils which are water soluble are likewise stored in this manner.
More commonly, however, large volume storage tanks of this nature are used in the production, collection and refining of crude oils and derivatives thereof such as crude oils containing naphthenic and aromatic components, and refinery products such as gasolines, diesel fuels, jet fuels, fuel oils, kerosene, gas oil, etc., and petrochemical derivatives thereof such as benzene, xylene, toluene, etc.
With the passage of time, solid materials, usually in finely divided form, will accumulate in the storage tank and settle at the bottom thereof. When the accumulation becomes excessive, it must be removed from the storage tank.
One manner in which this can be accomplished is to drain the tank and then have workmen enter the tank and manually remove the sediments that are deposited therein. However, such a procedure is costly and time-consuming and can cause the workmen involved therein to be exposed to toxic or potentially toxic materials.
The problem of sediment accumulation is particularly accentuated insofar as the storage of crude oil and, in particular, aromatic and naphthenic crude oils is concerned. Such crude oils, as introduced into the storage tank, will normally contain aromatic, napthenic and asphaltic components which are believed to be potentially reactive and/or condensible with each other. Moreover, a minor amount of water will normally be present in the crude oil (e.g., about 0.1 to 5 wt. %). Usually, the water will not be present as a separate phase, but rather as small droplets of water emulsified by ionizable components of the crude oil, such as asphaltenes.
It is believed that molecular charge transfer forces, such as van der waals forces, cause many of the molecular aromatic, napthenic and asphaltic components of the crude oil to agglomerate and weakly bond to each other to form aggregates having a size sufficient to cause them to precipitate from the crude oil and to settle at the bottom of a crude oil storage tank together with the emulsified water droplets so that the resultant "hydrocarbon sludge" will normally comprise highly aromatic components such as polyaromatic components in which a significant portion of the water (in the form of emulsified droplets) will be occluded. Also, when phyrins are present, the porphyrin molecules are believed to be attracted to each other so as to form agglomerates that will settle from the crude oil stored in the crude oil storage tank. It is for reasons such as these that the sediment in the bottom of a crude oil storage tank is sometimes colloquially referred to as "black sediment and water" or "hydrocarbon sludge" or just plain "sludge".
The hydrocarbon sludge that accumulates, as such, is of marginal economic value and, if manually removed, usually represents a disposal problem.
Various prior art methods have been suggested for removing such materials from storage tanks. For example, U.S. Pat. No. 1,978,615 to Erdman is directed to method and apparatus for cleaning sediment from a tank containing a fluid comprising a central manifold from which a plurality of discharge pipes radiate, each discharge pipe being provided with a plurality of discharge nozzles so that liquid may be pumped through the central manifold and out through the nozzles to roil the sediment or other foreign materials at the bottom of the tank and suspend it for withdrawal through a side withdrawal pipe located above the apparatus.
U.S. Pat. No. 2,116,935 issued to Richard et al. is directed to a method and apparatus for cleaning tanks such as railroad tank cars and comprises a pipe which is suspended vertically in the tank for rotation about a horizontal axis and which contains, at a lower end thereof, a reaction nozzle mounted for rotation about a horizontal axis and includes a reaction nozzle member mounted on vertical conduit for rotation about a horizontal axis so that liquid pumped down the conduit is forced out the vertically disposed jets of the reaction nozzle. The device also includes appropriate means for slowly rotating the reaction nozzle about the vertical axis of the suspending pipe.
U.S. Pat. No. 3,586,294 to Strong is directed to a method and apparatus for creating a suspension of fine particles in a liquid in a tank using a plurality of spargers suspended above the bottom of the tank on a nonrotating lattice of feed pipes through which a liquid is pumped for emission through the sparging nozzles to suspend fine particles of sediment in the liquid for discharge from the tank on removal of the suspension.
U.S. Pat. No. 3,878,857 to Heibo is directed to a device for cleaning the side walls of a storage tank such as a tank located on a ship carrying crude oil. The apparatus comprises an L-shaped inlet pipe suspended from the top of the tank. A pair of diametrically opposed jets are mounted on the end of the "L" so that liquid pumped through the L-shaped inlet pipe will be forced to flow out of the pipe through one of the jets at a time. Means are provided for rotating the jetting means a fraction of a turn about a horizontal axis for each complete revolution about the vertical axis. The mechanism for accomplishing this is a worm gear which operates in conjunction with a cog wheel and a blocking wheel.
U.S. Pat. No. 3,953,226 to Edmond et al. is directed to a device for cleaning sediment from a tank and includes pipe means oscillatably suspended from the top of the tank. The oscillatable pipe means is provided, at a discharge point near the bottom of the tank, with one or more spray jets through which hot water may be sprayed to sweep suspended matter to a sump located on the opposite side of the storage tank for removal.
U.S. Pat. No. 4,407,678, issued to Furness et al., discloses a sludge removal machine for removing sludge from the bottom of a storage tank which comprises a hollow body, and laterally rotatable nozzles. The sludge removal machine is suspended in a storage tank from a pipe through which a cleaning liquid may be pumped. The sludge removal machine is also provided with a "turbine" or impeller for rotating the nozzles in order to disperse sludge. The rotational speed of the turbine, and thus the rotation rate of the nozzles, is determined by the viscosity, pressure, and flow rate of the liquid pumped through the machine. Therefore, if it is desired to increase or decrease the fairly critical speed of rotation of the nozzles, one of these parameters, e.g., flow rate must be adjusted accordingly.
U.S. Pat. No. 4,685,974, also to Furness et al., is directed to a method for removing settled sludge from the bottom of a storage tank which uses apparatus of the type disclosed and claimed in Furness et al. A liquid such as crude oil is pumped into a machine suspended in a storage tank adjacent a side wall thereof and which is provided with diametrically opposed lateral nozzles which are rotated in a manner such that each nozzle emits liquid during 180.degree. of its rotation to avoid impingement of liquid on the side of the tank wall to thereby suspend the sludge in liquid in the tank, after which the liquid having sludge suspended therein is pumped from the tank.
In an improvement over the prior art, U.S. Pat. Nos. 4,945,933 and 5,019,016 disclose an apparatus useful for dispersing sediment contained in a crude oil storage tank. The apparatus generally comprises an oil circulator having a plurality of rotatably mounted nozzled outlet jets. Crude oil is continuously forced through the jets, whose rotation is controlled by an independently controllable indexing power means, to disperse the sediment.
While the prior art practices do provide methods for dispersing sludge in a crude oil storage tank, these methods require removal of the entire circulation apparatus from the tank which typically necessitates emptying of the tank before such removal can be accomplished. Therefore, there is a need for a method for dispersing sludge by rotating nozzles at a speed that is independently controllable of the fluid flow rate through the nozzles, and a method for easily removing the nozzle rotation drive train service or repair without emptying the storage tank or removing the entire fluid circulator.